Conventionally, divalent europium-activated phosphors with Eu.sup.2+ luminescent centers, such as BaMgAl.sub.10 O.sub.17 :Eu.sup.2+ or (Ba, Sr)MgAl.sub.10 O.sub.17 :Eu.sup.2+, Mn.sup.2+, are used for blue phosphors in luminescent devices.
Divalent europium-activated phosphors show emission spectra with a broad half-width that is due to the electron transition from the excitation level 4f.sup.6 5d to the ground level 4f.sup.7 characteristic to the Eu.sup.2+ ion. Moreover, in such phosphors, the ground level 4f.sup.6 5d is susceptible to the influence of the crystal field, so that, depending on the phosphor host, colors from ultra-violet spanning the entire visible range can be emitted.
Conventionally, divalent europium-activated phosphors are produced by reacting a mixture of the raw material for the phosphor, which has been mixed in a mixer such as a ball mill, in a reducing atmosphere (for example a gaseous atmosphere of nitrogen mixed with hydrogen). This process is described in more detail in "Keikoutai Handobukku" (The Phosphor Handbook) pp. 207-240, published by Ohmsha Ltd.
Chemical compounds comprising divalent europium ions are chemically unstable at regular temperatures and pressures in air. A compound comprising a trivalent europium such as Eu.sub.2 O.sub.3 can be used as the raw material for the europium of divalent europium-activated phosphors.
As a raw material for the divalent europium-activated aluminate phosphor with the above-mentioned structural formula, alkaline-earth carbonates (barium carbonate, strontium carbonate, basic magnesium carbonate, etc.) aluminum oxide, europium oxide, or manganese carbonate, mixed with a suitable amount of flux, can be used. Conventionally, divalent europium-activated aluminate phosphors are produced by (a) pre-firing this raw material for the phosphor in air at less than 1500.degree. C. as necessary, and then (b) firing it at 1200.degree. C.-1800.degree. C. for several hours in a reducing atmosphere (see for example the above-cited phosphor handbook, Publication of Examined Japanese Patent Application No. Hei7-77126, or J. Electrochem. Soc., Vol. 123, No. 5, pp. 691-697).
Thus, divalent europium-activated aluminate phosphors are conventionally produced by processing a raw material for the host compound serving as the phosphor host and a trivalent europium compound in a reducing atmosphere at a high temperature, directly or after pre-firing in air at a temperature that is lower than the firing temperature for the firing in a reducing atmosphere that follows.
The flux is added to promote the chemical reaction among the raw materials. For the flux, for example, a halogenide, such as aluminum fluoride, barium fluoride, and magnesium fluoride, can be used.
Divalent europium-activated phosphors other than aluminate phosphors include halophosphate phosphors such as Sr.sub.10 (PO.sub.4).sub.6 Cl.sub.2 :Eu.sup.2+, phosphate phosphors such as SrMgP.sub.2 O.sub.7 :Eu.sup.2+, silicate phosphors such as Ba.sub.3 MgSi.sub.2 O.sub.8 :Eu.sup.2+, and acid fluoride phosphors such as SrB.sub.4 O.sub.7 F:Eu.sup.2+.
Also these other divalent europium-activated phosphors usually can be produced by firing the raw material for the phosphor in a reducing atmosphere for several hours once or after firing for several hours in air at a temperature that is lower than the firing temperature for the firing in a reducing atmosphere that follows (see "The Phosphor Handbook").
However, in this conventional method for producing a divalent europium-activated phosphor, there is the problem that in addition to the desired divalent europium-activated phosphor, also divalent europium-containing phosphor is generated. Only a small amount of this phosphor is intermingled in the mixture, but since it luminesces together with the desired divalent europium-activated phosphor, the overall luminescent color purity of the divalent europium-activated phosphor worsens.
The mixed amount of divalent europium-containing phosphor varies among the production lots and according to the furnace used, so that it is difficult to control variations of the luminescent color of the phosphor. In particular, the excitation level 4f.sup.6 5d of the Eu.sup.2+ ion is susceptible to influences of the crystal field, so that even a tiny variation of the structure or crystallinity of the phosphor can change the excitation level a little. Thus, variations of the luminescent color are a particular problem with divalent europium-activated phosphors emitting a high-purity blue. However, a manufacturing method that effectively suppresses the intermingling of divalent europium-containing phosphor, has yet to be conceived.
For example, when a divalent europium-activated phosphor emitting a high-purity blue is produced with the conventional method, divalent europium-containing phosphor fluorescing in a highly visible green region is also generated.
To give a specific example, when a BaMgAl.sub.10 O.sub.17 :Eu.sup.2+ phosphor (emitting blue with a peak at 450 nm) is produced with the conventional method, a small amount of BaAl.sub.2 O.sub.4 :Eu.sup.2+ phosphor, which is a divalent europium-containing intermediate phosphor, is intermingled. This phosphor has an emission peak in the green region at 500 nm, so that the color purity of the blue phosphor is severely worsened.
As shown in FIGS. 16(a), 16(b) and 16(c), the x- and y-value of the CIE color coordinates for the blue phosphor produced with the conventional method vary considerably among different production lots (particularly the y-value). For comparison, FIGS. 16(a), 16(b) and 16(c) also shows the variations in the luminance of the blue phosphor. FIGS. 16(a), 16(b) and 16(c) indicates that in the conventional production method, the luminance also varies considerably.
The results in FIGS. 16(a), 16(b) and 16(c) show the emission characteristics for a Eu.sup.2+ phosphor excited with 254 nm ultraviolet light.